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What Is Carb Flu, & How Do You Treat It?

Lindsay Tigar is an experienced, established wellness, lifestyle, travel and love writer, editor and content strategist in New York City. Her work has appeared on SELF, Prevention, AskMen, Refinery29 and dozens of other sites. When she's...

If you start feeling ill after cutting carbs, this might be why

When you find yourself sniffling and coughing as the weather goes from rays of sunshine to crisp fall breezes, you might head to your doc for an annual flu shot. Though, sure, a change in temperature can cause an infection, you can also get flu-like symptoms from a transformation in your diet. Surprisingly, some of the setbacks from restricting a specific type of food can be detrimental, not only to your metabolism but to your overall health and vitality too.

One of the more common ailments many dieters suffer from is known as carb flu — which Dr. Keith-Thomas Ayoob, the associate clinical professor at Albert Einstein College of Medicine, says is actually better called "low-carb flu." Just as the name suggests, this is related to how many or how few carbohydrates you’re consuming. Here, experts shed light on what carb flu is and why you should be wary of it when attempting to improve your eating habits or drop pounds.

More: Is Low-Carb Better Than Low-Calorie Dieting?

What is carb flu?

Ayoob adds that in addition to being called the low-carb flu, you might also hear these same symptoms called "keto flu." When might you expect to experience this? When you go on a very low-carb diet, cutting out even good-for-you carbohydrates from all meals and supplements. This causes your body to perform differently and work double-time to function.

“You can feel a variety of symptoms that mimic the flu, from headache and lethargy and feeling weak to constipation or diarrhea,” Ayoob explains. You might also suffer from a brain fog, where you can’t seem to concentrate or focus on the task at hand. While he stresses that you won’t have a fever from the carb flu, it can wreak havoc in your day-to-day performance at work.

If you start feeling ill after cutting carbs, this might be why
Image: Getty/Design: Ashley Britton/SheKnows

What causes it?

Because all bodies need the appropriate balance of fat, carbohydrates, proteins and other essential nutrients, restricting one group to an extreme puts your body into a different mode of processing. As Ayoob explains, when you begin to contract the carb flu, it’s your body going into ketosis, a state in which it burns primarily fat for calories (hence the keto in keto flu). This is actually a prehistoric way for your body to process what you eat that started back when food was harder to come by and our early ancestors were forced to perform on much less intake.

“Fat is harder for the body to burn, and while it can survive on ketones, it’s definitely not desirable for the long-term. It’s how the body learned to survive during periods of famine and lack of regular food,” Ayoob says.

More: When Is the Best Time to Get Your Flu Shot?

Is it dangerous?

Ayoob says that more likely than not, carb flu isn’t life-threatening, with symptoms only lasting several days or a week. However, lingering conditions like bad breath and constipation can continue to manifest if you restrict your carbs consistently, mainly due to the lack of fiber in your diet. Ayoob’s best advice is to not spend too long on a low-carb crash diet and instead invest in a healthy alternative to losing weight.

“Your breath may smell rather acidic because you aren’t getting enough fiber, and ketones cause this stench,” he explains. “While not terribly dangerous for otherwise healthy people, ketosis is not where you want your body to be for any length of time.”

What happens when you have carb flu?

If you’ve been pushing away all breads, wraps and potatoes, you might notice for the first few days, you feel fine because your body still has glycogen, which is the store of carbohydrates your body reserves just in case you aren't getting enough food (or start a fad diet). However, Ayoob explains this stockpile only lasts for a few days before your body starts to burn mostly fat and protein, which is a tougher task. That’s why you start experiencing those exhausted feelings almost immediately. Depending on how much you want to stomach, Ayoob reminds us that coming off of a low-carb diet won’t instantly give you a boost of energy.

“You may still feel somewhat lethargic until you up your carb intake, even if not to where it was. Increasing carbs to 40 percent of your diet would prevent carb flu,” he says.

More: 7 Yoga Poses for When You Have the Flu

The bottom line

Ayoob says even in the search for the ideal filter-free body or shape, carbs still have a place in every diet. Because of the recent movement against carbs, this essential part of our diets has fear surrounding it.

“You’re leaving out whole food groups if you avoid carbs. Beans, whole grains, fresh fruits and vegetables, they’re all carb-rich but healthful and loaded with antioxidants and nutrients,” he says. “People can lose weight from a super-low-carb diet, but there’s a better way... without placing severe limits or even eliminating nutritional food groups. Aim for balance instead.”

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